Atomic Fountain Clock Research Articles

Switching atomic fountain clock microwave interrogation signal and high-resolution phase measurements

This paper focuses on the development of tools aiming to solve several problems related to the microwave interrogation signal in atomic fountains. We first consider the problem related to cycle synchronous phase transients caused by the sequential operation of the atomic fountain. To search for such systematic phase variations deeply buried in the microwave synthesizer phase noise, we have developed a novel triggered-phase transient analyzer capable of processing the microwave signal to extract the phase in a synchronous manner even in the presence of frequency modulation. With this device we check in vivo the LNE-SYRTE fountain's interrogation signals with a resolution approaching 1 microradian. In addition, using this device, we investigate an innovative approach to solve a second problem, namely, the shift caused by microwave leakage in the fountain. Our approach consists of switching off the fountain microwave interrogation signal when atoms are outside the microwave cavity. To do that, we have developed a switch that is almost free of phase transients and is thus able to eliminate the frequency shift caused by microwave leakage without inducing significant phase transients on the interrogation signal.

Ultralow noise microwave generation with fiber-based optical frequency comb and application to atomic fountain clock

We demonstrate the use of a fiber-based femtosecond laser locked onto an ultrastable optical cavity to generate a low-noise microwave reference signal. Comparison with both a cryogenic sapphire oscillator (CSO) and a titanium-sapphire-based optical frequency comb system exhibit a stability of about 3×10−15 between 1 and 10 s. The microwave signal from the fiber system is used to perform Ramsey spectroscopy in a state-of-the-art cesium fountain clock. The resulting clock is compared to the CSO and exhibits a stability of 3.5×10−14τ−1/2.

Absolute Frequency Measurement of theCa+404s S1/22−3d D5/22Clock Transition

We report on the first absolute transition frequency measurement at the 10;{-15} level with a single, laser-cooled 40Ca+ ion in a linear Paul trap. For this measurement, a frequency comb is referenced to the transportable Cs atomic fountain clock of LNE-SYRTE and is used to measure the 40Ca+ 4s ;{2}S_{1/2}-3d ;{2}D_{5/2} electric-quadrupole transition frequency. After the correction of systematic shifts, the clock transition frequency nu_{Ca;{+}}=411 042 129 776 393.2(1.0) Hz is obtained, which corresponds to a fractional uncertainty within a factor of 3 of the Cs standard. In addition, we determine the Landé g factor of the 3d;{2}D_{5/2} level to be g_{5/2}=1.200 334 0(3).

5D optics for atomic clocks and gravito-inertial sensors

A new framework is proposed to compare and unify photon and atomoptics, which rests on the quantization of proper time. A common waveequation written in five dimensions reduces both cases to 5D-optics ofmassless particles. The ordinary methods of optics (eikonal equation, Kirchhoff integral, Lagrange invariant, Fermat principle, symplectic algebraand ABCD matrices,...) are used to solve this equation in practical cases.The various phase shift cancellations, which occur in atom interferometers, and the quantum Langevin twin paradox for atoms, are then easily explained.A general phase-shift formula for interferometers is derived in fivedimensions, which applies to clocks as well as to gravito-inertial sensors.The application of this formula is illustrated in the case of atomicfountain clocks.

Ultra-Slow Atomic Beam Generation by Velocity Selective Resonance

We describe a method to generate an ultra-slow atomic beam by velocity selective resonance (VSR). A VSR experiment on a metastable helium beam in a magnetic field is presented and the results show that the transverse velocity of the deflected beam can be cooled and precisely controlled to less than the recoil velocity, depending on the magnitude of the magnetic field. We extend this idea to a cold atomic cloud to produce an ultra slow 87Rb beam that can be used as a source of an atomic fountain clock or a space clock.

拉曼喷泉原子钟

拉曼喷泉原子钟

An experimental study of intermodulation effects in an atomic fountain frequency standard

The short-term stability of passive atomic frequency standards, especially in pulsed operation, is often limited by local oscillator noise via intermodulation effects. We present an experimental demonstration of the intermodulation effect on the frequency stability of a continuous atomic fountain clock where, under normal operating conditions, it is usually too small to observe. To achieve this, we deliberately degrade the phase stability of the microwave field interrogating the clock transition. We measure the frequency stability of the locked, commercial-grade local oscillator, for two modulation schemes of the microwave field: square-wave phase modulation and square-wave frequency modulation. We observe a degradation of the stability whose dependence with the modulation frequency reproduces the theoretical predictions for the intermodulation effect. In particular no observable degradation occurs when this frequency equals the Ramsey linewidth. Additionally we show that, without added phase noise, the frequency instability presently equal to 2×10-13 at 1 s, is limited by atomic shot-noise and therefore could be reduced were the atomic flux increased.

Majorana Transitions in an Atomic Fountain Clock

Recently, the authors observed quasi-periodic frequency oscillations in CSF1, which is the first cesium fountain clock at Physikalisch-Technische Bundesanstalt, Braunschweig, Germany. These oscillations could be traced back to Majorana transitions after the second Ramsey interaction. A change in field configuration eliminated the Majorana transitions and with it the frequency oscillations

Normal-mode splitting with large collective cooperativity

We report the observation of normal-mode splitting of the atom-cavity dressed states in both the fluorescence and transmission spectra for large atom number and observe subnatural linewidths in this regime. We also implement a method of utilizing the normal-mode splitting to observe Rabi oscillations on the $^{87}\mathrm{Rb}$ ground state hyperfine clock transition. We demonstrate a large collective cooperativity, $C=1.2\ifmmode\times\else\texttimes\fi{}{10}^{4}$, which, in combination with large atom number, $N\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{5}$, offers the potential to realize an absolute phase sensitivity better than that achieved by state-of-the-art atomic fountain clocks or inertial sensors operating near the quantum projection noise limit.

Continuous beams of cold atoms for space applications

The precision of atomic state measurements should ideally be limited by irreducible fluctuations associated with the number of atoms available, e.g. shot noise or quantum projection noise. In practice, other noise sources can limit the precision achievable; a well-known effect is the intermodulation effect, or Dick effect, which degrades the stability of atomic fountain clocks using pulsed sources. One way to beat this source of instability consists of interrogating the atoms with a microwave signal derived from exceptionally stable local oscillators, such as cryogenic oscillators, which are however bulky and not ideally suited to the constraints of space. Another way of reaching atomic-noise limited instability is to use continuous sources of cold atoms. Experimental results obtained both on a fountain standard and on an experimental Cs fountain illustrate the potential of continuous cold atomic beams for improving signal-to-noise ratio and precision of measurement. Current developments towards higher atomic beam flux are also described.

Cold Atom Clock Test of Lorentz Invariance in the Matter Sector

We report on a new experiment that tests for a violation of Lorentz invariance (LI), by searching for a dependence of atomic transition frequencies on the orientation of the spin of the involved states (Hughes-Drever type experiment). The atomic frequencies are measured using a laser cooled 133Cs atomic fountain clock, operating on a particular combination of Zeeman substates. We analyze the results within the framework of the Lorentz violating standard model extension (SME), where our experiment is sensitive to a largely unexplored region of the SME parameter space, corresponding to first measurements of four proton parameters and improvements by 11 and 13 orders of magnitude on the determination of four others. In spite of the attained uncertainties, and of having extended the search into a new region of the SME, we still find no indication of LI violation.

Optical molasses loaded from a low-velocity intense source of atoms: an atom source for improved atomic fountains

We demonstrate the efficient capture of cesium atoms from a low-velocity intense source (LVIS) of atoms into an optical molasses. The high load rates that are achievable with the technique can potentially improve the stability of and reduce collisional shifts in atomic fountain clocks. An LVIS is an atomic beam created from a small hole in one of the retroreflectors in an otherwise ordinary magnetooptical trap (MOT). Our typical LVIS flux was 10/sup 10/ atoms/s. The asymptotic value for the number of atoms captured in the optical molasses was 1.1(1) /spl times/ 10/sup 9/ atoms, and the fill time constant was /spl tau/ = 290(30) ms. The initial molasses fill rate was R/sub t=0/ = 3.8(5) /spl times/ 10/sup 6/ atoms/ms. At this rate, it would require 24 ms to capture and launch 10/sup 7/ state-selected atoms in an atomic fountain. The fill rate at short times indicates that approximately 40% of the LVIS atoms were being captured in the optical molasses.

Atomic fountain clocks

We describe and review the current state of the art in atomic fountain clocks. These clocks provide the best realization of the SI second possible today, with relative uncertainties of a few parts in 1016.

Stabilized frequency comb with a self-referenced femtosecond Cr:forsterite laser

A frequency comb is generated with a Cr:forsterite femtosecond laser, spectrally broadened through a highly nonlinear optical fiber to span from 1.0 to 2.2 ,m, and stabilized using the f-to-2f self-referencing technique. The repetition rate and the carrier-envelope offset frequency are stabilized to a hydrogen maser, calibrated by a cesium atomic fountain clock. Simultaneous frequency measurement of a 657-nm cw laser by use of the stabilized frequency combs from this Cr:forsterite system and a Ti:sapphire laser agree at the 10(-13) level. The frequency noise of the comb components is observed at 1064, 1314, and 1550 nm by comparing the measured beat frequencies between cw lasers and the supercontinuum frequency combs.

Absolute-frequency measurements with a stabilized near-infrared optical frequency comb from a Cr:forsterite laser.

A frequency comb is generated with a chromium-doped forsterite femtosecond laser, spectrally broadened in a dispersion-shifted highly nonlinear fiber, and stabilized. The resultant evenly spaced comb of frequencies ranges from 1.1 to beyond 1.8 microm. The frequency comb was referenced simultaneously to the National Institute of Standards and Technology's optical frequency standard based on neutral calcium and to a hydrogen maser that is calibrated by a cesium atomic fountain clock. With this comb we measured two frequency references in the telecommunications band: one half of the frequency of the d/f crossover transition in 87Rb at 780 nm, and the methane v2 + 2v3 R(8) line at 1315 nm.

Proposal of a Novel Compact Cold Atomic Fountain Clock

We propose a novel atomic fountain clock which iscompact and transportable. The clock is 60-cm in height. Itslinewidth is expected to be 1.75 Hz, with accuracy10−14, stability 10−14 and signal–noise ratio103. A hollow beam is applied as atomic wave-guide, which canincrease the stability and the signal–noise ratio by one order. Thehollow beam will not affect the accuracy of the clock, if the frequencyshift due to the hollow beam is amended and the fluctuation of thehollow beam intensity is controlled to be smaller than 0.6%.

Fundamental physics research aboard the international space station

NASAs research plans aboard the International Space Station (ISS) are discussed. Experiments in low temperature physics and atomic physics are planned to commence in late 2005. Experiments in gravitational physics are planned to begin in 2007. A low temperature microgravity physics facility is under development for the low temperature and gravitation experiments. The facility provides a 2 K environment for two instruments and an experimental lifetime of 4.5 months. Each instrument will be capable of accomplishing a primary investigation and one or more guest investigations. Experiments on the first flight will study non-equilibrium phenomena near the superfluid 4He transition and measure scaling parameters near the 3He critical point. Experiments on the second flight will investigate boundary effects near the superfluid 4He transition and perform a red-shift test of Einstein's theory of general relativity. Follow-on flights of the facility will occur at 16 to 22-month intervals. The first couple of atomic physics experiments will take advantage of the free-fall environment to operate laser cooled atomic fountain clocks with 10–100 times better performance than any Earth based clock. These clocks will be used for experimental studies in General and Special Relativity. Flight definition experiment studies are underway by investigators studying Bose Einstein Condensates and use of atom interferometers as potential future flight candidates.

Towards an indium single-ion optical frequency standard

A narrow transition of a single laser-cooled ion, stored in a radio-frequency trap,can serve as a reference for a frequency standard of very high accuracy andstability. For the implementation of such a system, we study the 5s2 1S0–5s5p 3P0transition in In+, at a wavelength of 237 nm. This resonance has a linewidth of 0.8Hz, with systematic frequency shifts expected to be at the mHz level. A fractionalresolution of 1.3 × 10−13 has been so far achieved, limited by the frequency instabilityof the clock laser used to excite the line. The absolute frequency of the 1S0–3P0transition was measured using the frequency comb of a mode-locked fs laser, andas a reference a methane-stabilized He–Ne laser at 3.4 μm, which was calibratedagainst an atomic caesium fountain clock. The transition frequency wasdetermined as 1267 402 452 899.92 (0.23) kHz, the uncertainty being limited by theuncertainty of the He–Ne standard. The short-term frequency stability of theclock laser was recently greatly improved. With a new laser set-up a linewidth < 4Hz (FWHM) for integration times up to 26 s was achieved, using laserplatforms which actively isolate from external mechanical vibrations.

Design and realization of the microwave cavity in the PTB caesium atomic fountain clock CSF1

At the Physikalisch-Technische Bundesanstalt (PTB), the caesium atomic fountain clock CSF1 was developed. One key element of it is its microwave cavity, which was designed to have a low transversal phase variation across the cavity opening. This usually is achieved by using a cylindrical cavity with a TE011 field mode, having a high intrinsic quality factor, and by feeding the cavity symmetrically via two apertures at opposite position in the cavity wall. In contrast to other solutions, the CSF1 cavity is tightly coupled to the microwave feeds (Qload approximately 2000). Therefore, detuning of the cavity resonance frequency has a reduced impact on the microwave field amplitude in the cavity compared to the case of weak coupling. Thus, a temperature stabilization of the cavity can be avoided. The extent to which the tight coupling may have an impact on the transversal phase distribution was studied. This question was solved analytically for a simplified cavity model. The results were applied to define the coupling geometry of the CSF1 cylindrical cavity in such a manner that the transversal phase distribution should become minimized. The key parameter is the electric length of the waveguides that constitute the junction to the coupling apertures. The theoretical studies are presented in some detail, and steps of the practical realization of the cavity are described.

Optical frequency standards and measurements

We describe the performance characteristics and frequency measurements of two high-accuracy high-stability laser-cooled atomic frequency standards. One is a 657-nm (456-THz) reference using magneto-optically trapped Ca atoms, and the other is a 282-nm (1064-THz) reference based on a single Hg/sup +/ ion confined in an RF-Paul trap. A femtosecond mode-locked laser combined with a nonlinear microstructure fiber produces a broad and stable comb of optical modes that is used to measure the frequencies of the reference lasers locked to the atomic standards. The measurement system is referenced to the primary frequency standard NIST F-1, a Cs atomic fountain clock. Both optical standards demonstrate exceptional short-term instability (/spl ap/5/spl times/10/sup -15/ at 1 s), as well as excellent reproducibility over time. In light of our expectations for the future of optical frequency standards, we consider the present performance of the femtosecond optical frequency comb, along with its limitations and future requirements.